Fail-safe circuit arrangement

ABSTRACT

A fail-safe signal absence detection circuit conditioned by the presence of input signals for establishing one of two conductive states of a trigger and a switching circuit for maintaining an oscillating circuit in a nonoscillating condition wherein an output signal is not produced and conditioned by the absence of input signals for alternately establishing the two conductive states of the trigger and the switching circuit for permitting the oscillating circuit to assume an oscillating and a nonoscillating condition wherein an output signal is produced.

O United States Patent 1 13,553,488

[ 72] inventors John 0. G. Darrow [56] References Cited Murrysville; UNITED STATES PATENTS l N 58;;23 2,762,464 9/1956 Wilcox 331/65 g gg 1968 3,015,077 12/1961 Elam et a1 331/65 Patented Jan. 3,147,408 9/ 1964 Yamamoto et al. 331/65 [73] Assignee Westinghouse Air Brake Company Primary Dnald Forrer Swissvale, Pa. Assistant Examiner- Harold A. Dixon a corporation of PennsyIvania Attorneys-W. I... Stout and John B. Sotak ABSTRACT: A fail-safe signal absence detection circuit conditioned by the presence of input signals for establishing one [54] E SE Q of two conductive states of a trigger and a switching circuit for n g maintaining an oscillating circuit in a nonoscillating condition [52] U.S. Cl 307/231, wherein an output signal is not produced and conditioned by 307/246, 307/290; 328/67, 328/5; 331/65 the absence of input signals for alternately establishing the two [51] Int. Cl G081) 21/00 conductive states of the trigger and the switching circuit for [50] Field of Search 328/1, 5; permitting the oscillating circuit to assume an oscillating and a 331/65; 307/246, 290, 231 nonoscillating condition wherein an output signal is produced.

i I I 15 {l6 l i I 15) f I I 12 Q l 17 19 51 I j: I 6 5?: 1&6 I 66 7 7 I 1 F I It i 22 1% g 1 T I 91 41 1 l l L J 2 L 4 Q 2 c 1 ramssrscmcurr ARRANGEMENT Our invention relates to a fail-safe signal absence detection circuit and more particularly to an electronic zero level detector which operates in a fail-safe manner to provide an output when and only whenan input is absent. c

' In various'control apparatus, such as an automatic vehicle speed control system for massand/or rapid transit operations, safety is of paramount importance. However, such security demands that each vital portion or section of an automatic speed control system forrapid and/or mass transit operations must function in a fail-safe manner in order to preclude damage to equipment and, to prevent injury to individuals. This type of system security notonly must be practiced in the moving or routing of the vehicles but also must be exercised at the station stops so that the highest degree of safety may be afforded the general public. For example, in routing, every precaution must be taken to prevent collisions between vehicles, and, in stopping,.additional measures must be taken to insure that a 7 vehicle is not moving duringthe loading and unloading of the passengers. Accordingly, it is essential and necessary to'absolutely insure that under no-circumstances will the vehicle doors be inadvertently opened'when the vehicle is in motion. For example, prior to opening the doors for discharging the passengers at a stationplatform the vehicle must come to a complete stop and under no circumstance should the doors be i s will indicate that a movingveh'icle is at rest. That is, the detection apparatusregardless of its operating condition must not produce an output when the vehicle is in motion. Therefore, in designing the motion detection apparatus, it is of utmost importance to carefully analyze and thoroughly evaluate the circuitry and every component thereof from every conceivable angle to makecertain that a componentfailure or circuit malfunction will not result in an output which is capable of simulating an untrue condition;

By our standard which is in keeping with the A.A.R. definition, a piece of apparatus 'or' equipment is considered to operate in a fail-safemanner when any conceivable component or circuit failure will result in a condition at least as restrictive or as safe as that preceding the failure. However, in order to insuresuch security, it is necessary to accurately resolve the wiring arrangement for preventing short circuits from existing between unrelated wires or leads and to carefully analyze the various types of electrical and electronic components, both active and passive elements, from the standpoint of partial failure due to erratic changes in their normal operating characteristic as well as complete failure due to short-circuited and open-circuited conditions. In addition, special consideration must also be given to open circuits caused by mechanical failure of an electrical connection as well as to output from amplifiers caused by extraneous pickup, power supply ripple or feedback oscillation. Accordingly, failsafe designs require careful circuit scrutiny and analyzation.

Therefore, it is an object of the present invention to provide a new and improved signal absence detection circuit.

It is another objectof the present invention to provide an improved signal absence detection circuit which operates in a fail-safe manner.

It is still another object of the present invention to provide a fail-safe electronic zero level detector which is incapable of producing an output signal during the presence of an input signal. q

A further object of the present invention is to provide an improved signal absence detection circuit which is incapable of producing an unsafe output signal during a malfunctional condition.

Still another object of the present invention is to provide a fail-safe signal absence detection. circuit which provides an output signal only during the absence of an input signal.

Yet still another object of the present invention is to provide a transistorized signal absence detection circuit which operates in a fail-safe manner and which produces an output signal only during the absence of an input signal.

Still another object of the present invention is to provide a fail-safe circuit arrangement which is simple to construct, feasible to use, efficient and reliable in operation.

Broadly and in accordance with the present invention we provide a novel signal detection circuit having means responsive to the presence of an input signalfor preventing the production of an output signal and means responsive to the absence of the input signal for providing the production of an output signal. More specifically, we provide a unique fail-safe circuit arrangement comprising an oscillating circuit and first circuit means coupled to the oscillating circuit for rectifying andstoring a predetermined potential charge when the input signal ispresent, or when the oscillating circuit is oscillating and the input signal is absent. The first circuit means is coupled to a trigger circuit which assumes a first stable conductive condition whenever the. predetermined potential charge is developed and which assumes a second stable conductive condition whenever the predetermined potential charge is decreased. A second circuit means coupled to the trigger circuit for maintaining the oscillating circuit in a nonoscillating condition whenever the trigger circuit is in its first stable conductive condition and for causing theoscillating circuit to oscillate whenever the trigger circuit is in its second stable conductive condition. And a third circuit means coupled to the trigger circuit for only providing an output signal whenever the triggering circuit alternately assumes its second and first stable conductive conditions due to the absence of the input signal. 7

The foregoing objects and other attendant features and advantages of this invention will become more fully evident from the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is a schematic circuit diagram of a signal absence detection circuit embodying the present invention and FIG. 2 is a schematic circuit diagram of a modification of the signal absence detection circuit shown in FIG. 1.

Referring now to the drawings wherein like components are designated by the same reference characters and, more particularly to FIG. 1, there is shown a zero level detector or signal absence detecting circuit embodying the present invention. In general, the schematic circuit. illustrated in FIG. 1 comprises an oscillator l, a first amplifier 2, a rectifier-filter 3, a trigger circuit 4, a switching circuit 5, a second amplifier 6, and a rectifier 7. As will be described in greater detail hereinafter, each of these various circuits are operatively interconnected to accomplish the purposes of the present invention.

As shown, the oscillator 1 is preferably but not necessarily a Colpitts-type oscillator comprising a transistor amplifier 9 and a suitable resonant tank circuit 10. The amplifier 9 includes a PNP transistor T1 having emitter, collector and base electrodes. The base electrode 12 of transistor T1 is connected to the junction of a voltage divider which consists of resistors 13 and 14 connected in series between conductor 8 and common input conductor 11. The emitter electrode 15 of transistor T1 is connected by resistor 16 to conductor 8, the potential level of which is controlled by the switching circuit 5, as will be described in greater detail hereinafter. The resonant tank circuit 10 which determines the frequency of oscillation comprises an inductance coil 18 and a pair of capacitors 19 and 20 which are suitably interconnected between the input and output circuits of transistor amplifier for providing the necessary regenerative feedback.

In a motion detection arrangement for mass and/or rapid transit operations, it has been found convenient to employ a generating device which is suitably associated with the axle and wheels of a vehicle for producing discrete signals in response to the movement thereof. While it is apparent that the signal generating device can be any one of several types available such as, a permanent core and coil arrangement which is influenced by the teeth of a conventional gear, there is shown diagrammatically in FIG. 1, an electromagnetic pickup device employing a permanent magnet type of gear or toothed wheel 22 which is in inductive relationship with the inductor coil 18 of the resonant tank circuit 10. The toothed wheel 22 may be suitably connected, for example, through a gear train, to the axle of the vehicle wheels so that movement of the vehicle causes rotational movement of the wheel 22 and, in turn, moves the gear teeth in relation to the inductor 18. It will be appreciated that rotational movement of the geared wheel causes alternating current voltage signals to be induced in the inductor coil 18. Further, it will be appreciated that the frequency of the alternating current signals generated in coil 18 is proportioned to the speed of the toothed wheel,

which in turn is proportional to the speed of the moving vehicle. As is readily obvious, when the vehicle is not moving or stopped, rotational movement of the gear 22 also ceases so that no alternating current voltage is induced in the inductive coil 18. Thus, alternating current voltage signals induced in the coil 18 may be interpreted as the input signal which is present only when the vehicle ismoving. Accordingly, a

necessary control function, such as, a door opening operation, may be based upon the absence of signals induced in the coil 18, as will be described in greater detail hereinafter.

As shown, the inductor 18 of the tank circuit as well as the output circuit of the oscillator 1 is coupled to the input of the two stage amplifier 2. That is, the collector 17 of transistor T1 and the upper terminal of the inductor 18 are coupled to the. base electrode of the first stage NPN transistor T2 by resistor 24. The emitter electrode 26 of transistor T2 is directly connected to the common input conductor 11 while the collector electrode 27 of transistor T2 is connected to the base electrode 28 of the second stage PNP transistor T3 by resistor 29. The emitter electrode of transistor T3 is directly connected to the positive terminal +V of a suitable source direct current operating voltage (not shown) while the collector electrode 31 of transistor T3 is connected to the common input conductor 11 by resistor 32.

The output of the second stage of transistor amplifier 2 is coupled to the input of the rectifier-filter circuit 3. The rectifi- ,er-filter circuit 3 is made up of coupling capacitor 35, halfwave rectifying diode 36, resistors 37, 38 and 39 and a fourterminal storage capacitor 40. The input to this circuit is taken from the collector electrode 31 of transistor T3 and fed while the associated lower terminal of capacitor 40 is connected to the common input conductor 11. The other lower terminal of the four-terminal capacitor 40 is connected to the common output conductor 41 while the other of its two upper terminals is connected to the resistor 39, which forms the input circuit to the Schmidt trigger 4. It will be appreciated that by employing a four-terminal capacitor rather than a con- I .ventional type of capacitor the possibility of causing an unsafe condition due to a capacitor lead or leads becoming accidentally disconnected is eliminated.

The Schmidt trigger 4, as is well known, is a regenerative bistable whose circuit conductive condition or state is dependent upon the amplitude of the input voltage. As shown, the trigger circuit is made up of two NPN transistors T4 and T5. The base electrode 44 of transistor T4 is connected to the resistor 39 while the emitter electrodes 45 and of transistors T4 and T5, respectively, are connected to the common output conductor 41 by common-emitter resistor 46. The collector electrode 47 of transistor T4 is connected to the base elecpositive terminal +V of the potential supply source.-

As shown, the switching circuit 5 simply comprises an NPN transistor T6 having its base electrode '57-directly connected to thecollector electrode 47 of transistor T4. The collector electrode 58 of transistor T5 is directly-connected to the positive terminal +V of the supply source while-its emitter electrode 59 is connected to the conductor 8'..-As willbe described in greater detail hereinafter, the conductive state of the switching circuit 5 is controlled in accordance; with the conductive condition of the Schmidt trigger 4. l

As shown, the output amplifier 6 consistsofa twostage transistor amplifier. The first stage preferably employs. a. PNP

transistor T7 arranged in a grounded emitter configuration while the second stage employs an NPN transistorT8 airanged as an emitter-follower. The base electrode '60 of transistor T7 is connected via resistor 61 to the collector electrode 51 of transistor T5. The emitter electrode 62,- of transistor T7 is directly connected to the positive terminalj-i-V of the supply source. The 'collector electrode 63 of transistor T7 is connected to the common output connector 41 by resistor 64 and also directly connected to the base electrode 65 of transistor T8. The collector electrode 66 of transistor T8 is directly connected to the positive terminal +V of the supply source. The emitter electrode 67 of transistor T8 is connected to the anode of diode 68 whose cathode is connected to the common junction of collector 63, base electrode 65, and the upper terminal of resistor 64. The function of diode 68 will be described in greater detailhereinafter. The output from amplifier 6 is coupled to the input of the fail-safe rectifier 7. j

The rectifier 7 is made up of a pair of capacitors 70 and 71 and a pair of diodes 72 and 73 forming a voltage doubling network. The emitter electrode 67 of transistor T8 is connected by capacitor 70 to the junction point of the anode of diode 72 and the cathode of diode rectifier 73. The cathode of diode rectifier 72 is connected to the common output conductor 41. The anode of diode 73 is connected to one side of the capacitor 71 and also forms the output terminal 75 which may be connected to a suitable utilization device, as will be described hereinafter. The other end of capacitor 71 is connected to the common output conductor 41.

Turning now to the operation, it will be assumed that the vehicle is in motion and proceeding between stations so that the axle coupled toothed wheel 22 is rotating and inductively producing AC signals in the coil 18. As previously mentioned, the generated signals have a frequency which is dependent upon the speed of the vehicle. These signals are applied to the base electrode 25 of the first stage transistor T1 of amplifier 2 and appear as amplified signals at the collector electrode 31 of transistor T3. The amplified signals taken from the collector 31 of second stage transistor T3 are, in turn, applied to the rectifier which only permits the passage of positive half cycles of the amplified signals. The rectified signals are, in turn, delivered by the ;resistor 37 to the four-terminal capacitor 40 which offers a; lowimpedance to AC ripple frequency and a high impedance to the DC component for filtering and smoothing purposes. As a result, the rectified signals charge the capacitor 40 to substantially the positive peak value of the signals within a given period of time which is determined by the RC time constant of the charging circuit. The positive charge on capacitor 40 is, in turn, delivered to the base electrode 44 of transistor T4 through resistor 39. It is noted th at a sufficient positive potential, dependent upon the response level of the Schmidt trigger, on the base electrode 44 causes NPN transistor T4 to conduct which, in turn, results in a regenerative action to take place until transistor T4 is operating in the saturation region and the transmitter T5 becomes completely cut off. Accordingly, the Schmidt triggcr,4 assumes one of its two stable conductive conditions at this time, namely, transistor T4 conducting heavily and transistor T5 nonconducting.

trode 47 of transistor T4 of the trigger circuits so that the conductive condition of the switch is controlled in accordance with the conductive state of the Schmidt trigger 4. That is, when the trigger circuit 4 is in its first stable state, namely, transistor 4 ON and transistor 5 OFF, the switching circuit assumes its first or low conducting condition so that a relatively 3 available at the collector electrode 51 of transistor T5 so that no input signal is applied to the amplifier 6. Accordingly, the amplifier 6 is in a state of quiescence and no output signal will appear on emitter electrode 67 of transistor T8. With no signal applied to the input of the rectifier 7, no output voltage will be produced on terminal 75. It will be noted that this condition will prevail so long as the train is in motion and input signals are induced in coil 18 due to the rotation of the permanent magnet wheel 22. Further, so long as the Schmidt trigger 4 remains in its first stable condition due to the potential charge on capacitor 40, the switching circuit 5 is held in its low conducting state sothat the oscillating circuit 1 remains inactive.

Let us now assume that the vehicle is approaching a station platform so that the vehicle is under deceleration and slowing down to come to a stop for unloading and loading passengers. Under this condition, the rotation of thepermanent magnet wheel 22 is correspondingly decreased and when the vehicle is sufficiently slowed down and stopped,signals will no longer be constants of the discharge path, the potential charge on capacitor 40 begins to decrease to a level determined by the characteristics of the Schmidt trigger. Accordingly, when the positive potential on base electrode 44 of transistor T4 becomes sufficiently reduced, the Schmidt trigger 4 undergoes a conductive change of state wherein transistor T5 turns ON while transistor T4 turns OFF. The turning OFF of transistor T4 causes the switching circuit 5 to undergo a switching change wherein transistor T6 begins to conduct heavily. The turning ON of the switching transistor T6 causes the potential level on emitter electrode 59 and, in turn, on conductor 8 to rise and to assume a potential value substantially equal to the voltage now present on the collector electrode 47 of transistor 14. That is, the conduction of transistor T4 allows sufficient operating potential to be applied to the Colpitts oscillator 1 so that the necessary bias conditions of the transistor are met and the oscillations occur. Accordingly, AC voltage signals are produced on collector 17 of transistor T1 which, in turn, are delivered to the base electrode of transistor T2 by resistor 24. These AC signals are, in turn, amplified and appear at the collector 31 of transistor T3. These amplified signals are, in turn, rectified by diode 36 and again begin to recharge the capacitor 40 through resistor 37. After a predetermined period of time, the capacitor again assumes a sufficient positive charge to cause conduction of transistor T4 of the Schmidt trigger 4. That is, the positive charge on capacitor 40 is again applied to the base 44 of transistor T4 via resistor 39 thereby causing a transistor T4 to turn ON which, in turn, causes transistor T5 to turn OFF. The turning ON of transistor T4 causes the switching circuit 5 to undergo a change of state wherein the transistor T6 again exhibits a high impedance condition so that the voltage on emitter 59 and, in turn, on conductor 8 drops to a relatively low potential level. With the necessary operating potential removed from conductor 8, amplifying qualities of the transistor T1 are destroyed, and oscillation ceases and AC signals are not produced on the collector electrode 17. The cessation of the oscillations causes the disappearance of the amplified signals from the collector 31 of transistor T3. Thus, with the removal of the signals from the collector electrode 31, the capacitor 40 again begins to discharge through the previously mentioned discharge path. When the capacitor 40 is sufficiently discharged, the transistor T4 again turns OFF while the transistor T5 again turns ON. The turning OFF off transistor 4 again causes the switching transistor T6 to assume its low impedance high conduction condition so that sufficient operating potential is again available on conductor 8. The increased voltage on conductor 8 suitably biases the transistor oscillator Tl so that oscillations occur and AC signals again appear on the collector electrode 17 of transistor T1. These AC signals are again amplified by the amplifier 2 and appear on the collector electrode 31 of transistor T3. The amplified signals again are rectified and begin to charge up the capacitor! which when sufficiently charged causes the transistor T4 to turn ON wherein transistor T6 turns OFF and removes the operating potential from the oscillator 1 which in turn ceases to oscillate. It will be appreciated that this cycle of operation is repeated so long as no input signals are induced in the inductor 18 by the permanent magnet wheel 22. Accordingly, the transistor T4 of the Schmidt trigger 4, the switching transistor T6, the conductor 8, the oscillator l, amplifier 2, and rectifier-filter 3 form a loop which undergoes a cyclic conductive change due to the absence of input signals being induced in the conductor 18 while the vehicle is not in motion. It will be appreciated that the frequency of oscillation of the loop is chosen to be sufficiently low in regard to the lower frequency of the input signals produced by the magnetic pickup device in order to prevent the generated input signals from causing activation of the cyclical loop operation.

Further, it will be noted that the recycling loop operation causes the Schmidt trigger and particularly the transistor T5 to alternately conduct and thereby produce pulsing or squarewave signals appearing on the input of the amplifier 6 are suitably amplified and appear on the emitter electrode 67 of transistor T8. These amplified square-wave signals are, in

' turn, applied to the voltage rectifier 7. The half wave voltage doubler rectifies the amplified signals so that a negative output signal or potential appears on the terminal 75 at this time.

In analyzing the operation of the voltage doubler 7, it will be noted that during the maximum value of the square waves, the capacitor is charged through the diode rectifier 72 to common output conductor 41. Conversely, during the minimum value of the square wave signals a discharge path is completed by diode 68 through resistor 64 to the capacitor 71 so that a negative potential having a value substantially equal to the magnitude of the square wave signals appears on the output terminal 75. In addition, it will be noted that the use of diode 68 improves the operation and avoids the need of employing a load resistor for the transistor T8. That is, a load resistor in an emitter-follower stage simply consumes power in the form of heat and thereby reduces the overall effectiveness and efficiency of the circuit.

As previously mentioned, the output voltage appearing on output terminal 75 may be employed to energize suitable utilization apparatus, such as an electromagnetic or static relay, or any other suitable switching or logic circuits for signifying that the vehicle is stopped or at rest. Accordingly, the vehicle door controlling mechanism may be actuated either manually by a vehicle attendant or automatically by suitable vehicle carried control apparatus when an output appears on terminal 75. It will be appreciated that'in the instant case it is preferred that the vehicle door opening operation will only be initiated when the voltage appearing on terminal 75 is of a proper polarity, namely, negative with respect to the normal operating potential so that it is not possible to falsely initiate a safe door opening signal from the supply source due to a circuit or component failure, as will be described in greater detail hereinafter.

-- Let us now assume that the passengers have been loaded so that the vehicle doors may be closed and the vehicle itself may proceed to its next scheduled stop. Movement of the vehicle again causes the AC signals to be induced in the inductor coil 18 which in turn are applied to the input of amplifier 2 and appear as amplified signals on the collector electrode 31 of transistor T3. The signals are in turn rectified by the diode 36 and either maintain the potential charge of capacitor 40 if the capacitor is already charged or begin charging the potential level of the capacitor if in its discharge state. In one case, the rectified signals will maintain a charge on capacitor 40 and in the other case the rectified signals will charge capacitor 40 to a sufficient potential level, to cause transistor T4 to go into conduction. As previously mentioned, when transistor T4 is conducting, transistor T is nonconducting. Also, the nonconducting condition of transistor T4 causes the switching circuit 5 to assume its low conductive condition wherein the operating potential level on emitter electrode 59 and, in turn, on

Accordingly, it will be noted that the presence of an input signal and the absence of an output signal signifies that the vehicle is in motion while the absence of an input signal and the presence of an output signal signifies that the vehicle is not in motion which in turn may be conveniently utilized to control the door opening operation in a mass and/ or rapid transit system in a fail-safe manner.

Further, it will be noted that by uniquely requiring a negative polarized output signal for signifying a vehicle at rest, a circuit or component failure, namely, a short circuit between the supply source and the output terminal 75 cannot simulate a vehicle at rest due to the reversal in polarity. However, it will be appreciated that a polar sensitive output device is not absolutely necessary for ensuring fail-safe operation since it would require a multitude of failures, namely, a short-circuited transistor T8, capacitor 70, diode 73 and an open-circuited diode 74 to provide a positive potential on terminal 75. In addition, it will be noted that a failure within the oscillating circuit 1 either results in the loss of the bias voltage require ment which, in turn, nullifies the necessary amplification characteristics on the transistor or destroys the required regenerative feedback necessary for sustaining oscillations. Further, it is noted that if the inductor l8 fails as either an open-circuited or short-circuited element, the frequency determining characteristics of the oscillator are similarly destroyed so that oscillations cannot be produced. In addition, it will be noted that a failure within either the amplifier 2 or 6 either results in the loss of the necessary biasing voltage to prevent proper operation or destroys the necessary amplifier characteristics of the active elements, themselves. A failure within the rectified filter 3 generally causes loss of rectified voltage for the capacitor 40. Similarly, a fault occurring within the Schmidt trigger 4 results in the inability of the circuit to produce the necessary AC or square wave signals for amplifier 6I'The switching circuit 5 is fail-safe from the standpoint that an open-circuited active element prevents operating voltage from being applied on conductor 8 and a short-circuited element causes the oscillator 1 to continually oscillate thereby simulating the presence of the input signals being generated in inductor 18 by the movement wheel 22. Further, any failure within the voltage rectifier 7, such as, a shortor open-circuited component, results in the total loss of the negative output voltage or signal. For example, a fault occurring in the rectifier cannot produce the necessary negative potential at the output terminal 75 for activating the output utilization device. As previously mentioned, it will be observed that since the main power supply is-positive and that by requiring a negative operating potential as a valid output signal, no circuit or component failure within amplifier 6 and rectifier 7 can produce a false indicating signal.

Referring now to FIG. 2 there is illustrated a variant of the circuit of FIG. 1 having a more general application. As shown, the toothed wheel 22 of the magnetic pickup device has been replaced by a suitable transformer an'd 'san' appropriate source of AC signals 82. The alternating current signal source 82 is directly coupled to the primary winding 81 .--Ihe inductor coil 18' of the oscillator tank circuit 10' functionsas the secondary winding of transformer 80 and feeds the input signals to the input of amplifier 2. As is readily evident, the remaining portions of the oscillator circuit are identical to that shown in FIG. 1 with the numeral characters simply primed for convenience. Further, the remaining circuits (not shown) which make up the overall signal absence detection circuit embodying the present invention may be identical to those shown in FIG. 1. While the AC signal source is shown to be transformer coupled to the inductor coil 18 and, in turn, to the oscillator and the input of amplifier 2, it is readily understood that the source of AC signals may be suitably interconnected or coupled to the circuit input in any other conventional and appropriate manner, such as, being directly coupled, capacitively coupled, resistively coupled, etc.

Accordingly, it can be seen that the presently-described signal absence detection circuit may be readily adapted for general usage to detect the absence of any type of AC input signals, in a fail-safemanner.

In addition, it is readily understood that the' complements'of the transistors, shown and described in the circuits may be employed and that the output voltage on terminal 75 may be of a positive polarity by simply reversing the polarity of the DC supply source and the diodes, as is well known, and also that the output of both the rectifier-filter and the rectifier should be of a polarity opposite to that of the DC power supply.

It will be appreciated that while this invention finds particular utility in a vehicle door control system, it is readily evident that this invention is not limited thereto but may be employed in other various systems and apparatus, such as, vehicle speed control arrangements employing overspeed and stop and proceed equipment as well as other apparatus, which require the safety and security inherently present in this invention. But regardless of the manner in which the invention is used, it is understood that various alterations may be made by persons skilled in the art without departing from the spirit and scope of this invention. It will also be apparent, that other modifications and changes can be made to the presently described invention, and, therefore, it is understood that all changes, equivalents and modifications within the spirit and scope of the present invention are herein meant to be included in the appended claims.

We claim:

1. A fail-safe circuit for detecting the presence and absence of an input signal comprising:

an oscillating circuit;

first means coupled to said oscillating circuit for rectifying and storing a predetermined voltage charge when said input signal is present, and when said oscillating circuit is oscillating but said input signal is absent;

an electronic regenerative trigger circuit coupled to said first means which assumes a first stable current conducting condition whenever said predetermined voltage charge is developed and which assumes a second stable current conducting condition whenever said predetermined voltage charge is sufficiently decreased;

second means coupled to said trigger circuit for maintaining said oscillating circuit in a nonoscillating condition whenever said trigger circuit is in said first stable current conducting condition and for causing said oscillating circuit to oscillate whenever said trigger circuit is in said second stable current conducting condition; and

third means coupled to said trigger circuit for only providing an output signal whenever said trigger circuit alternately assumes said second and said first stable current conducting conditions due to the absence of said input signal. v r

2. A fail-safe circuit as defined in claim I, wherein said first means includes a transistor amplifier, a diode rectifier and a four-terminal capacitor.

3. A fail-safe circuit as defined in claim 1, wherein said trigger circuit comprises a transistorized Schmidt trigger.

4. A fail-safe'circuit as defined in claim 1, wherein said oscillating circuit comprises a Colpitts-type oscillator.

5. A fail-safe circuit as defined in claim 1, wherein said oscillating circuit includes a tuned circuit having an inductor coil and split capacitors and having the input signal induced into said inductor coil.

6. .A fail-safe circuit as defined in claim 1, wherein said second means comprises a switching circuit having a firstand a second conductive conditionl 7. A fail-safe circuit as defined in claim 6, wherein said switching circuit controls the oscillating condition of said oscillating circuit by controlling the level of operating potential applied to said oscillating circuit.

8. A fail-safe circuit as defined in claim 1, wherein said third means includes an amplifier and a rectifier circuit.

9. A fail-safe circuit as defined in claim 8, wherein said rectifier circuit comprises a voltage doubler.

3,553,4ss I er-filter circuit, a switching circuit coupled to said bistable trigger circuit and assuming a first and a second conductive condition in accordance with the conductive conditions of said trigger circuits, an oscillating circuit intercoupled from said switching circuit to said first amplifier and controlled by the conductive condition of said switching circuit, a second amplifier circuit coupled to said bistable-trigger circuit, and a rectifier circuit coupled to said second amplifier circuit for providing an output when and only when the signals from said input source are absent.

12. A fail-safe circuit as defined in claim 11, wherein said second amplifier circuit comprises a first and a second transistor and includes a diode for interconnecting the emitter electrode to the base electrode of said second transistor.

13. A fail-safe circuit as defined in claim 11, wherein said rectifier circuit comprises a voltage doubler.

14. A fail-safe circuit as defined in claim 11, wherein said bistable trigger circuit comprises a Schmidt trigger which as sumes and remains in a first conductive condition during'the presence of said input signals. p

15. A fail-safe circuit as defined in claim 14, wherein said rectifier-filter circuit includes a four-terminal capacitor which maintains a sufficient voltage charge for holding said Schmidt trigger in its first conductive condition during the presence of said input signals.

16. A fail-safe circuit as defined in claim 15, wherein said switching circuit controls the oscillating condition of said oscillator circuit by controlling the level of operating potential applied to said oscillator circuit.

17. A fail-safe circuit as defined in claim 11, wherein said source of input signals comprises an electromagnetic pickup device. I

18. A fail-safe circuit as defined in claim 17, wherein said oscillator circuit includes inductance-capacitance tuning.

19. A fail-safe circuit as defined in claim 18, wherein said electromagnetic pickup device includes an inductor fonning the inductance of said inductance-capacitance tuning. 

1. A fail-safe circuit for detecting the presence and absence of an input signal comprising: an oscillating circuit; first means coupled to said oscillating circuit for rectifying and storing a predetermined voltage charge when said input signal is present, and when said oscillating circuit is oscillating but said input signal is absent; an electronic regenerative trigger circuit coupled to said first means which assumes a first stable current conducting condition whenever said predetermined voltage charge is developed and which assumes a second stable current conducting condition whenever said predetermined voltage charge is sufficiently decreased; second means coupled to said trigger circuit for maintaining said oscillating circuit in a nonoscillating condition whenever said trigger circuit is in said first stable current conducting condition and for causing said oscillating circuit to oScillate whenever said trigger circuit is in said second stable current conducting condition; and third means coupled to said trigger circuit for only providing an output signal whenever said trigger circuit alternately assumes said second and said first stable current conducting conditions due to the absence of said input signal.
 2. A fail-safe circuit as defined in claim 1, wherein said first means includes a transistor amplifier, a diode rectifier and a four-terminal capacitor.
 3. A fail-safe circuit as defined in claim 1, wherein said trigger circuit comprises a transistorized Schmidt trigger.
 4. A fail-safe circuit as defined in claim 1, wherein said oscillating circuit comprises a Colpitts-type oscillator.
 5. A fail-safe circuit as defined in claim 1, wherein said oscillating circuit includes a tuned circuit having an inductor coil and split capacitors and having the input signal induced into said inductor coil.
 6. A fail-safe circuit as defined in claim 1, wherein said second means comprises a switching circuit having a first and a second conductive condition.
 7. A fail-safe circuit as defined in claim 6, wherein said switching circuit controls the oscillating condition of said oscillating circuit by controlling the level of operating potential applied to said oscillating circuit.
 8. A fail-safe circuit as defined in claim 1, wherein said third means includes an amplifier and a rectifier circuit.
 9. A fail-safe circuit as defined in claim 8, wherein said rectifier circuit comprises a voltage doubler.
 10. A fail-safe circuit as defined in claim 8, wherein said amplifier comprises a first and a second transistor in which a diode interconnects the emitter and base electrodes of said second transistor.
 11. A fail-safe circuit arrangement comprising, a source of input signals, a first amplifier circuit coupled to said input source for amplifying said input signals, a rectifier-filter circuit coupled to said first amplifier circuit for rectifying and filtering said amplified signals, a bistable trigger circuit having a first and a second conductive condition coupled to said rectifier-filter circuit, a switching circuit coupled to said bistable trigger circuit and assuming a first and a second conductive condition in accordance with the conductive conditions of said trigger circuits, an oscillating circuit intercoupled from said switching circuit to said first amplifier and controlled by the conductive condition of said switching circuit, a second amplifier circuit coupled to said bistable trigger circuit, and a rectifier circuit coupled to said second amplifier circuit for providing an output when and only when the signals from said input source are absent.
 12. A fail-safe circuit as defined in claim 11, wherein said second amplifier circuit comprises a first and a second transistor and includes a diode for interconnecting the emitter electrode to the base electrode of said second transistor.
 13. A fail-safe circuit as defined in claim 11, wherein said rectifier circuit comprises a voltage doubler.
 14. A fail-safe circuit as defined in claim 11, wherein said bistable trigger circuit comprises a Schmidt trigger which assumes and remains in a first conductive condition during the presence of said input signals.
 15. A fail-safe circuit as defined in claim 14, wherein said rectifier-filter circuit includes a four-terminal capacitor which maintains a sufficient voltage charge for holding said Schmidt trigger in its first conductive condition during the presence of said input signals.
 16. A fail-safe circuit as defined in claim 15, wherein said switching circuit controls the oscillating condition of said oscillator circuit by controlling the level of operating potential applied to said oscillator circuit.
 17. A fail-safe circuit as defined in claim 11, wherein said source of input signals comprises an electromagnetic pickup device.
 18. A fail-safe circuit as defined in claim 17, wherein said oscillator circuit includes inductance-capacitance tuning.
 19. A fail-safe circuit as defined in claim 18, wherein said electromagnetic pickup device includes an inductor forming the inductance of said inductance-capacitance tuning. 